The invention relates to a pneumatic sheet feeder.
U.S. Pat. No. 3,964,740, for example, discloses a pneumatic sheet feeder which may be used in a copying machine or facsimile system. The feeder comprises a sheet receptacle on which a stack of sheets is placed, a vacuum casing located above the sheet receptacle at a location forwardly as viewed in the sheet feed direction and having an open bottom to define a vacuum chamber, a single movable bottom plate which is pivotally mounted on the vacuum casing on a horizontal axis at its rear end and including an air suction aperture or apertures adjacent its forward end and which is adapted to close the bottom opening of the vacuum chamber, and a sheet feed roller or rollers disposed within the vacuum chamber. A negative pressure is created within the vacuum chamber, and an uppermost sheet of the stack placed on the receptacle is held attracted to the underside of the bottom plate while the latter moves angularly to permit the feed roller or rollers to project through part of the openings to feed the sheet forwardly by friction.
In a sheet feeder of this kind, the movable bottom plate usually comprises a single plate member pivotally mounted on a horizontal axis disposed at the rear end of the casing and extending in a direction transverse to the sheet receptacle. When the negative pressure is not created within the vacuum chamber, the front end of the bottom plate bears against the uppermost sheet of the stack by weight. Consequently, the movable bottom plate assumes a varying angle of inclination relative to the uppermost sheet as the number of sheets contained in the stack or the height thereof changes. When the negative pressure is created, there occurs an airflow which is directed through the air suction openings formed in the bottom plate into the vacuum chamber, and the uppermost sheet is attracted and held attracted against the lower surface of the bottom plate to close the openings, thus maintaining the vacuum chamber substantially air-tight to permit the negative pressure within the chamber to be increased. When the negative pressure increases above a given value, the bottom plate moves angularly in the upward direction about the pivotal axis, whereby the uppermost sheet is separated from the next lower sheet. In order to assure a stabilized sheet separation and feeding operation, it is desirable that the configuration of the space defined by the lower surface of the bottom plate and the uppermost sheet remains unchanged throughout, namely, from the time the initial sheet is fed until the last sheet in the stack is fed from the sheet receptacle. However, with pneumatic sheet feeders of the conventional design, the bottom plate will assume an increasing angle of inclination relative to the uppermost sheet in the stack as the height thereof decreases, resulting in a change in the configuration of the space defined by the bottom plate and the uppermost sheet.
When the sheets are tightly stacked, or when there is no substantial air layer between adjacent sheets, more than one sheet may be attracted simultaneously, fouling the desired separation. If more than one sheet is fed in superposed relationship in a facsimile system, a jamming occurs, causing a failure of the system. Additionally, it will be appreciated that when an airflow occurs from the rear end of the feed rollers toward the air suction openings, the sheet separating air stream, that is, the flow of air around the front edge of the uppermost sheet into the space between the uppermost sheet and the next following sheet, is impeded, thus adversely influencing the sheet separation. In this manner, it will be appreciated that a close control of the sheet separation is required.
When the sheet feeder of the type described is used with the input section of a facsimile system, the feed rollers of the feeder deliver an original to be transmitted into the input section. Thereupon, the feeding operation of the original is continued by an original feeder of the facsimile system, which feeds it into the processing station. It will be understood that it is desirable, for purpose of reducing the noise and the power dissipation, that the feed operation by the sheet feeder be interrupted when the sheet or the original can be fed by the original feeder. In this instance, if the original is elongate in the feed direction, when the feed operation of the feeder is interrupted, the downwardly moving bottom plate may clasp the sheet being fed, thus increasing a load on the original feeder and causing an adverse influence upon the read operation. In worst cases, the original may be damaged.
In the sheet feeder disclosed in the above-mentioned U.S. Pat. No. 3,964,740, the feed rollers are carried by the vacuum casing and project downwardly through the air suction openings formed in the bottom plate when the latter moves angularly through a given stroke in the upward direction. This disadvantageously prevents a stabilized sheet separation in that the front end of the uppermost sheet is kept from upward movement by the front end of the movable bottom plate.
To avoid this difficulty, the present applicant has proposed a sheet feeder in U.S. application Ser. No. 825,389, now abandoned, in which the feed rollers are mounted on the bottom plate so as to press against the sheets on the receptacle. However, in this sheet feeder, the rotation is transmitted to the feed rollers by friction with a drive shaft which is arranged to be moved into sliding contact with the feed rollers when the feed rollers are displaced upwardly through a given stroke together with the bottom plate. This requires a biasing pressure of a relatively large magnitude between the feed rollers and the drive shaft in order to assure the stabilized rotation of the feed rollers. Consequently, a large proportion of the fan capacity which produces the negative pressure to lift the bottom plate upwardly must be reserved for use as the bias pressure. Hence, the fan becomes less effective in lifting the bottom plate, with a reduced holding effect of the feed roller. When the sheet holding effect of the feed roller is reduced, the uppermost sheet will be bent under the action of the negative pressure beyond a boundary defined by the feed rollers, and the bottom plate may begin to move upwardly before the uppermost sheet is held attracted to the lower surface of the bottom plate, thus resulting in an unstabilized sheet separation. In addition, when the feed rollers are mounted on the bottom plate, the overall weight of the latter increases, presenting a problem that an increased negative pressure is required to lift it upwardly.